Iterator for a subset of a vector

Question

Is it possible to get a const iterator from a vector that can only iterate a certain range of the vector before being invalidated?

For example if I have a vector of 10 elements, I want to return an iterator of elements 4 to 7.

pseudo-code:

int main()
{
    std::vector<int> vector = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

    auto iterator = GetRangedIterator(vector, 4, 7)
    for (const int& num : iterator)
        print num;      // 4, 5, 6, 7
}

Answer 1

This is pretty trivial to do (though I'd call the result a range, not an iterator).

A simple implementation would look something like this:

template <class Iter>
class range {
    Iter b;
    Iter e;
public:

    range(Iter b, Iter e) : b(b), e(e) {}

    Iter begin() { return b; }
    Iter end() { return e; }
};

template <class Container>
range<typename Container::iterator> 
make_range(Container& c, size_t b, size_t e) {
    return range<typename Container::iterator> (c.begin()+b, c.begin()+e);
}

As it stands right now, this follows normal C++ conventions (0-based counting, the end you specify is past the end of the range, not in it) so to get the output you asked for, you'd specify a range of 3, 7, like:

for (int num : make_range(vector, 3, 7))
    std::cout << num << ", ";      // 4, 5, 6, 7,

Note that the range-based for loop knows how to use begin and end member functions to tell it the range it's going to iterate, so we don't have to deal with invalidating iterators or anything like that, we just have to specify the beginning and end of the range we care about.

Answer 2

You could use the range-v3 library that is the bases of a Ranges TS which will been part of C++20, but the library already works with C++11 compilers. Here's how:

#include <range/v3/all.hpp>
#include <iostream>
#include <vector>

int main() 
{
    using namespace ranges;

    auto v = view::iota(1, 11) | to_<std::vector<int>>();
    std::cout << view::all(v)  << '\n';

    auto rng = v | view::slice(3, 7); 
    std::cout << rng << '\n';
}

Live Example.